Drupal Evolved for several decades and gradually became the preferred framework for websites around the world, known as a Content Management System, it became the core system for companies and organizations around the world. Many companies invested heavily into tailoring Drupal Versions into their own system, most of whom were based on version 7 of Drupal.
Then came Drupal 8, which promised to integrate everything everyone ever wanted into the core CMS system, and the number of lines of code in the core system jumped from 1700 lines of code to 24000 lines of code, which was pretty efficient code, but viewed as more overhead than desired by those users who didn't want to undertake a rewrite of their systems that they had already written so many sites elected to freeze things at Drupal7.
But the combination of 100 time the original number of lines of code, against the cost of supporting the core system backfired with many companies. None of them wanted to spend 100 times as much supporting features they had already decided they didn't need.
Drupal 9 and Drupal 10 both have people pushing their development, but many of those are hoping to produce the version of the framework that can become the next Drupal 7. One of those drivers is the number of companies who decided not to migrate away from Drupal 7, mostly because they had invested enough already and didn't want to invest more.